What went wrong?

Russians mobilized faster than expected

German Troops were taken away from the right hook to deal with Russia at Tannenberg.

Germany somewhat stupidly believed that Britain wouldn't get involved at all, thinking they were only going to be fighting the French.

French and British troops were able to lay down defensive networks at the Marne fast enough to stop the advance.

>>1698722
I was watching a documentary about this the other week.

As first post said, Russians mobilized much faster than the Schlieffen plan was hoped for. Also, General Moltke altered Schlieffen's original plan which Schlieffen warned would leave the right flank exposed.

>>1698864
Schlieffen's last words to Moltke were "don't forget to strengthen the right flank."

>>1698722
>Russia mobilized faster, causing 1 German army to be used defensively in East Prussia (Tannenberg)
>Strong Belgian resistance
>Belgian soldiers destroyed their own infrastructure to slow the Germans down
>Schlieffen plan was already outdated

They didn't actually execute the Schlieffen Plan.